Strontium chroma ie light-sensitive



United States Patent 3,172,763 STRONTIUM CHROMATE LIGHT-SENSITIVE CARBONTISSUE Charles A. Kumins, Chappaqua, and Daniel Sm th, Riverdale, N.Y.,and Rudolf G. Frieser, Neshanrc Station, N.J., assignors toInterchemical Corporation, New York, N.Y., a corporation of Ohio NoDrawing. Filed May 12, 1961, Ser. No. 109,511 4 Claims. (Cl. 96--75)This invention relates to the manufacture of so-called carbon tissuesemployed in the photomechanical reproduction art. These tissues areemployed in preparing etched printing plates or rolls from copy whichmay be a photographic transparency of a line drawing or of a many-tonedsubject. They are also of use in preparation of gelatin reliefs for usein printing by the imbibition processes. This application is acontinuation-in-part of our application Serial No. 683,969, filedSeptember 16, 1957.

Up to the time of our present invention, it has been the practice to usecarbon tissues consisting essentially of light sensitive coated papers,such as light sensitive gelatin coated papers. The sensitized coatingmay contain minor amounts of other ingredients, such as pigment (to makethe relief image more readily visible and to control penetration ofactinic radiation into the gelatin layer) and such materials as soaps,sugar, glycerin, etc., to improve its physical properties. Becauselamp-black was formerly used as the pigment in coating such papers, theycame to be known as carbon tissues"; other pigments are now commonlyemployed, among them finely powdered iron oxide, such as jewelers rouge.

Carbon tissues as presently manufactured are quite stable in storage andessentially insensitive to light. Before use, however, it is necessaryto sensitize them by wetting them thoroughly with a sensitizing solutionof a salt of hexavalent chromium, usually a dilute solution of an alkalidichromate, such as ammonium dichromate, sodium dichromate, or potassiumdichromate. After being thoroughly wetted with sensitizing solution, thetissue must be dried and used rather promptly. The sensitivity of thedried tissue changes with storage, and tissue at least one day old ispreferred for carbon printing because the initial changes in sensitivityare so rapid and large as to produce results variable beyond the limitsof commercial tolerance. Storage for more than a week aftersensitization is impractical since, even without exposure to light, thegelatin hardens gradually, and can then no longer be differentiallyinsolubilized by exposure to light and then partially dissolved to forma satisfactory relief image.

The mode of employment of carbon tissues is familiar to thosewell-versed in the printing and photographic arts, and, since itconstitutes no part of our invention, will not be further describedhere, except insofar as necessary to describe our invention.

It is an object of our invention to provide a new type of coatingwhereby storage-stable, light-sensitive carbon tissues may be prepared.A further object is to provide a process of producing an emulsion ofextremely finely divided water-insoluble chromate pigments.

It is a still further object of our invention to provide a carbon tissuewhich is sensitized at the time of manufacture and which is stableenough in storage as to permit 3,172,763 Patented Mar. 9, 1965 it to bestocked ready for use by iobbers and the ultimate user. It is anotherobject to provide a method of producing such a carbon tissue. Otherobjects will become apparent from the detailed description of ourinvention which follows.

Surprisingly, we have found that if we coat paper or other flexiblewater-insoluble membrane with an aqueous solution of gelatin, or anequivalent sensitizable material, containing dispersed therethroughextremely small and uniform-sized particles of a substantiallywater-insoluble chromate, preferably strontium chromate, a tissue isobtained which has all the desirable properties of carbon tissue freshlysensitized by the previously known method and which, in addition iscapable of being stored before use for periods up to six months or evena year. The advantages of such a product are obvious, and the industryhas long sought in vain for such a material.

While we do not wish to be bound by or limited to any theory orattempted explanation as to the reactions involved in the practice ofour invention, we believe, on the basis of our study and experiments,that the effect of light on the chromated gelatin is to convert chromate(or bichromate) ion to chromic ion, and that the chromic ion so formedacts to denature (i.e., harden or insolubilize) the gelatin. Thesereactions appear to occur slowly even without the influence of lightwhen the chromate or bichromate ions are in solution, so that afterbrief storage the gelatin of a conventional carbon tissue sensitized inthe usual manner becomes hardened throughout and is no longer usable forthe preparation of a gelatin relief image. We have found that this rapiddeterioration of the sensitive material does not occur when the chromateis present as a water-insoluble solid.

In order to prepare satisfactory carbon tissues according to ourinvention, it is important that the insoluble chromate dispersed in thegelatin be in an extremely fine state of subdivision. This aids in theuniform distribution throughout the gelatin necessary for unifomiity ofthe coating, increases transparency, and makes for greaterphotosensitivity.

We have found that desirably fine-particle chromates can be prepared byprecipitation in an aqueous system of increased viscosity. Preferablyfor the purpose of preparing coatings for our novel carbon tissues, weemploy gelatin for increasing the viscosity of the reaction medium,though other thickening agents, including gum arabic, gum tragacanth,and other natural and synthetic thickening agents may be employed.

We have found it possible to employ the chromates of several differentmetals in the practice of our invention, including lead, silver,strontium, and barium. We have found strontium chromate to give tissuesof excellent sensitivity, shelf life, and scale-gradation, andaccordingly prefer it for use in our invention. For precipitating theinsoluble chromates for use in coating our novel carbon tissues, anysoluble salt of strontium, lead, silver, or barium may be used in thepractice of our invention, but for reasons of economy, availability, andconvenience, we prefer the nitrates. Similarly, the choice of a solublesalt of hexavalent chromium employed is not particularly critical. Foreconomy and convenience we prefer sodium dichromate, though ammoniumdichromate is equally suitable.

The particle size of the insoluble chromates dispersed in gelatinaccording to our novel process is very uniform and is generally about100-1000 Angstrom units. The

particles appear to be spheres, and so difier from the much largerrod-shaped particles in which lead and strontium chromates are obtainedby ordinary precipitation methods.

The following examples illustrate our invention.

Example I Fifty parts of unfiavored edible gelatin (Knoxs) and parts ofjewelers rouge are added to 250 parts of warm (60 C.) water, and stirreduntil a homogeneous mixture is obtained. This mixture is allowed to coolto 40 C. (while stirring is continued), and 19.1 parts of anhydrousstrontium nitrate is added. After the strontium nitrate is dissolved, asolution of 13.4 parts of sodium dichromate dihydrate in 250 parts ofwater is added slowly over a period of about an hour. The mixture isthen brought to a pH of between 7 and 8 by the addition of about 10parts of concentrated ammonium hydroxide solution. Stirring is continuedfor minutes, after which the emulsion is bottled, cooled to roomtemperature, and stored in the dark. From the time of commencing theaddition of dichromate to the time of bottling, the temperature of themixture is maintained at about C. and all operations are carried out inthe dark or in weak, diffused, preferably yellow, light. The emulsionmay be used at once for coating paper, or may be stored for severaldays. It is inadvisable to store the wet emulsion for a very long time,however, since it deteriorates on standing, and may become entirelyinactive if stored for a month at room temperature.

A bottle containing the emulsion prepared above is placed in a warmwater bath to heat its contents to about 40 C. and convert the gel to afiowable liquid. This liquid is fiowed onto a smooth-surfaced paper toproduce a uniform coating. The paper is then transferred promptly to acold environment to cause setting of the gelatin. (A household-type ofrefrigerator provides satisfactory cooling for small lots of tissue.)After setting of the gel, the tissues are dried in a current of air.Prompt cooling of the coated tissue is desirable so as to avoidpenetration of the paper, as this would interfere with later stripping.The speed of drying is somewhat less critical, but since the undriedcoating emulsion has only limited storage stability, it is preferably toreduce the water content rather promptly to assure uniformly sensitivetissues of good stability in storage. All operations in the preparationof the carbon tissue are conducted in weak, preferably yellow, light,but there is no need for total darkness; the working space may becomfortably lighted, so long as direct sunlight or intense artificiallight is avoided.

The tissue prepared above may be employed in the wellknown manner forthe preparation of gelatin washoff relief images for imbibition printingor resists for etching of printing plates or rolls. Exposure may be bysunlight, carbon, mercury vapor, or rare gas arcs, or by tungstenfilament lamps.

The exposed tissues may be transferred in the usual manner to polishedmetal, paper, Celluloids, or the like. The relief images may bedeveloped in the usual manner with warm water. Metal plates or rolls onwhich the relief images are developed may be etched with the usualetching solutions, such as aqueous ferric chloride.

Example II Nineteen grams of strontium nitrate are dissolved in asolution of 200 grams of gum arabic in 250 milliliters of water at 40 C.A solution of 13.4 grams of sodium dichromate dihydrate in 250milliliters of water is added dropwise to the strontium nitrate solutionand then 10 milliliters of ammonium hydroxide (28%) is added. Stirringis continued for 20 minutes, after which the emulsion is bottled, cooledto room temperature, and stored in the dark. From the time of commencingthe addition of dichromate to the time of bottling, the temperature ofExample III Using a procedure similar to that of Example 11 a chromntedemulsion was prepared from (A) 250 grams of a 7.5% aqueous solution ofcarboxy methyl cellulose gum (Hercules CMC, Type 7MS) described by thesupplier as a moderately low molecular weight material having aviscosity of 2 to 8 poises (2% solution in water at 25 C.) withapproximately 25% of the available groupings in the original cellulosesubstituted by carboxy methyl groups,

(B) 10 grams of strontium nitrate,

(C) 13.4 grams of sodium dichromate dihydrate in 250 milliliters ofwater, and

(D) 10 milliliters of ammonium hydroxide (28%).

A carbon tissue was prepared according to the procedure in Example IIand exposed to a carbon arc source through a conventional high contrastline negative. Five seconds exposure gave a well defined image, with theunexposed portions being readily removable by washing at F.

Example IV In a manner similar to that of Example II a light sensitiveemulsion was prepared from (A) grams of Elvanol 51-05 dissolved in 250milliliters of water. (Elvanol 5l-05 is described by the supplier as alow molecular weight polyvinyl alcohol having a viscosity of 4.2ccntipoises as measured in a 4% aqueous solution at 20 C., a ph of 5.9,and an 88.2% hydrolyzed polyvinyl acetate.)

(B) 19 grams of strontium nitrate.

(C) 13.4 grams of sodium dichromate dihydrate in 250 milliliters ofwater.

(D) 10 milliliters of ammonium hydroxide (28%).

This emulsion gave a light sensitive coating on paper of lesssensitivity than the emulsion of Example III in that it required about30 seconds exposure to develop an image as against five seconds for thecoating of Example III.

From the above examples it is evident that the above invention hasgeneral applicability to the preparation of light sensitized coatings ingeneral.

We claim:

1. A light-sensitive relatively storage stable carbon tissue comprisinga flexible, water-insoluble, supporting membrane, coated with a thin,substantially uniform light sensitive emulsion layer comprised of amaterial of the group consisting of gelatin, polyvinyl alcohol, carboxymethyl cellulose and gum arabic in which is dispersed a finely dividedstrontium chromate as a photosensitizing component.

2. A product according to claim 1 in which the average size of thestrontium chromate particles is less than 1000 Angstrom units.

3. A light-sensitive relatively storage-stable carbon tis sue comprisinga flexible, water-insoluble supporting membrane coated with a thin,substantially uniform layer of gelatin in which is dispersed a finelydivided strontium chromate as a photosensitizing component.

4. A product according to claim 3 in which the average size of thestrontium chromatc particles is less than 1000 Angstrom units.

(References on following page) 6 References Cited in the file of thispatent OTHER REFERENCES UNITED STATES PATENTS Canadian Patent OfficeRecord, vol. 74, 1946, page 428,472 Dub M r. 16, 1887 1479- 1,984 090,513,, 11, 1934 Handbook of Chemistry and Physics, Chemical Rubber 2 1 51 Izard June 13 1939 5 Publishing C0., Cleveland, 01110, 35th edition,1953-1954, 2,902,390 Bell Se t. 1 1952 Pages p Kuhne, Farbe and Lack:Centralblatt, 1930, pages 243- FOREIGN PATENTS 244 23 5 97,115 GermanyMar. 21, 1898 Tritton: The Photographic Journal, pages 281-285,

1,983 Great Britain Aug. 26, 1898 10 June 1929. of 1856 Kolthoff et al.:Volumetric Analysis, vol. 3, page 335.

435,337 Canada June 18, 1946 Interscience Publishers Inc., N. Y., 1957.

1. A LIGHT-SENSITIVE RELATIVELY STORAGE STABLE CARBON TISSUE COMPRISINGA FLEXIBLE, WATER-INSOLUBLE, SUPPORTING MEMBRANE, COATED WITH A THIN,SUBSTANTIALLY UNIFORM LIGHTSENSITIVE EMULSION LAYER COMPRISED OF AMATERIAL OF THE GROUP CONSISTING OF GELATIN, POLYVINYL ALCOHOL, CARBOXYMETHYL CELLULOSE AND GUM ARABIC IN WHICH IS DISPERSED A FINELY DIVIDEDSTRONTIUM CHROMATE AS A PHOTOSENSITIZING COMPONENT.